Capacitive touch sensors (such as touch buttons) are increasing used in human interface devices. These touch sensors usually have a base capacitance and function based on detection of an increase in the base capacitance due to the presence of a dielectric (i.e., finger) in proximity to the sensor. With some touch sensors, this change or variation in base capacitance (which can be referred to as the touch capacitance) can be as small as 0.5%. This means that, if a 10-bit successive approximation register (SAR) analog-to-digital converter (ADC) is employed to digitize the measurement, the touch capacitance measurement may be limited to approximately the 5 least significant bits. Thus, there is a high susceptibility to error due to noise. Additionally, the base capacitance can drift over time, which can create further errors. Therefore, there is a need for an improved touch controller that can accurately measure small touch capacitances.
Some examples of conventional systems are: U.S. Pat. No. 5,463,388; U.S. Pat. No. 7,764,274; U.S. Patent Pre-Grant Publ. No. 2007/0074913; U.S. Patent Pre-Grant Publ. No. 2008/0116904; U.S. Patent Pre-Grant Publ. No. 2009/0066674; U.S. Patent Pre-Grant Publ. No. 2009/0153152; U.S. Patent Pre-Grant Publ. No. 2010/0201382; and PCT Publ. No. WO2007044360.